Why's and When's of Monitor Calibration

Why's and When's of Monitor Calibration
Michael H. Brill, Principal Color Scientist, Datacolor

What is monitor calibration? It is the process of assigning a lookup table between two triplets of R, G, B digital command values: the command that once (or maybe on another display) produced a particular color, and the command that actually produces that color now on your monitor. Once you have calibrated your monitor, the same commands (e.g., from a digital photo) will produce the same colors reliably on your monitor.

How is a monitor calibrated? A set of command values are input, a colorimeter (such as the Spyder2PRO™) measures the resulting displayed colors, and software produces a lookup table that takes the color you get into an intended target color. Target colors are denoted by three values X, Y, Z, which are called tristimulus values and represent the eye's response to the light. The lookup table, called a profile, is stored when the monitor is turned off and re-installed when the monitor is turned back on. For continued color fidelity, the monitor profile should be updated regularly by re-calibration.

Why do I need to re-calibrate my monitor? The color that emerges from any digital command will drift over time. For different display technologies, this will occur for different reasons, but all displays undergo color drift. For CRTs, the drift comes from differential aging (hence dimming) of the red, green, and blue phosphors. Typically, the blue phosphor ages most quickly and the red phosphor has the longest life. Hence differential aging leads to the colors on a CRT shifting toward the red and away from the blue. This differential aging, which for CRTs is greatest when the monitor is nearly new, is quite obvious in the monitor white. Also, for CRTs the black and white level settings undergo drift with time [Planar, Calibration TQA User's Guide, Dome™ Imaging Systems, 2001, pages 5-6]. Differential aging analogous to that of CRT phosphors occurs with the multiple phosphors in the fluorescent backlight on certain liquid crystal displays (LCDs). Such effects are also present for light-emitting-diode (LED) backlights. It should also be noted that the light from an LED is sensitive to the temperature of the lamp, which can vary depending on a monitor's viewing environment. To preserve calibration, it is wise to keep the temperature as steady as possible.

For plasma displays, the story is similar, because such displays too are based on phosphors. Of course, plasma-display emission is mediated by UV excitation, unlike the electron-mediated emission in CRTs, but the story is the same. Ronald Peterson reports ["Materials challenges for flat panel displays," 204th Meeting, 2003 Electrochemical Society, Inc., www.electrochem.org/dl/ma/204/pdfs/1164.PDF] that the usual blue phosphor, BaMgAl10O17Eu2+, decreases its emission by more than 30 percent in 1000 hours of irradiation time. Organic light-emitting-diode displays have similar aging problems, to the point of impacting the effective lifetime of the displays. Although CRTs and LCDs on the market age less severely than these products, the aging problem is significant enough to require monitor calibration as we describe here.

The figure below shows the drift measured from an actual high-end studio LCD (courtesy of Dr. Colman Shannon at Datacolor). Within 50 days, the drift on all color coordinates became visually noticeable. Drift may be more pronounced on lower-end monitors.

How often do I need to re-calibrate my monitor? Once a month should be sufficient for casual purposes. (see, e.g., the Blacksburg Photo Club blog: http://wp.blacksburgphoto.org/?p=58). If you're a professional photographer who relies on trustworthy reproduction of monitor color in prints, more frequent calibration may be desirable---perhaps every one or two weeks. If the monitor undergoes marked temperature change, altitude or pressure change, or a mechanical shock, recalibration is also recommended. If you let calibration slide for a while, you may be shocked when you finally do calibrate, as this Australian testimonial attests: "When we clicked on the before and after image to see the effect of the adjustment we were appalled. How could we have let the monitor drift away from true colour for so long? We know that we should readjust every couple of weeks but we had left it for three months."